Syncengine:  Wait for the neon thead to be finished before destroying the Propagator and calling csync_commit

The legacy job might still need the neon session and the propagator.
We need to make sure the thread exits before.

This fixes crash when pausing a sync made with the legacy jobs
(for example when there is network limitation)
This should fix https://github.com/owncloud/enterprise/issues/200
Legacy propagator jobs: Do not limit bandwidth when aborting

When aborting, we want the last job to be as fast as possible
as it blocks the UI.  So don't limit the bandwidth in that case